1. Field of the Invention
This invention relates to zoom lenses of the rear focus type and, more particularly, to large relative aperture, high range zoom lenses of the rear focus type having a zoom ratio of 10 and an F-number of 1.8 or thereabout for use in photographic cameras, video cameras or cameras for broadcasting.
2. Description of the Related Art
For the photographic camera or video camera, a wide variety of zoom lenses having the focusing provision in a lens unit other than the first lens unit when counted from the object side, that is, employing the so-called rear focus type, have been proposed.
In general, the rear focus type, because of the focusing lens unit being relatively small in size and light in weight, has advantages in that a small driving power for the focusing lens unit is sufficient, and rapid focusing can be performed, etc.
For example, Japanese Laid-Open Patent Applications Nos. Sho 62-24213, Sho 63-29718, Hei 2-48621 and Hei 2-53017 have proposed the rear focus type of zoom lenses each of which comprises, from front to rear, a first lens unit of positive refractive power, a second lens unit of negative refractive power for varying the focal length, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, totaling four lens units, the first and third lens units being stationary, the second lens unit being moved to vary the focal length, while the fourth lens unit is simultaneously moved to compensate for the shift of an image plane, and focusing being performed by moving the fourth lens unit.
Particularly, in the above-mentioned Japanese Laid-Open Patent Application No. Hei 2-53017, an aspheric surface is used in the third lens unit so that the third lens unit is constructed with a single lens element. Thus, a zoom lens of the rear focus type whose total length is shortened to a compact form is proposed.
From the use of the rear focus type in the zoom lens, there are obtained, in the general case, the advantages that the bulk and size of the entire lens system are minimized, that a high-speed focusing becomes possible, and further that the focusing range becomes easy to extend toward shorter object distances.
On the other hand, however, the variation of aberrations with focusing is caused to increase. Thus, a problem arises in that it becomes very difficult to obtain a high optical performance from an infinitely distant object to a closest object, i.e., over all the object distances, in such a manner that the bulk and size of the entire lens system are minimized. Particularly, in the zoom lens of a large relative aperture and as high a range as 10 or thereabout in the zoom ratio, it becomes very difficult to obtain a high optical performance throughout the entire zooming range as well as the entire focusing range, giving rise to another problem.